The Reagan administration sounded a note of caution yesterday about the Soviet Union's new arms proposals as U.S. and Soviet negotiators sat down to discuss them in detail in Geneva.

White House spokesman Larry Speakes told reporters to "look very closely at what you hear and see concerning this offer, and particularly the 50 percent." The Soviet Union, in a proposal delivered to President Reagan last week, called for the superpowers to cut strategic "nuclear charges" in half.

Kenneth L. Adelman, director of the Arms Control and Disarmament Agency, called the Soviet proposals "a mixed bag."

"There are some positive notions, but many elements are blatantly one-sided for unilateral Soviet advantage," Adelman said in an interview.

Asked for examples of one-sidedness, Adelman said, "Clearly they want us to gut Strategic Defense Initiative research completely." At the same time, he charged, "they would continue their research, as they have for a good number of years" on defenses against nuclear missiles.

Regarding potential reductions, Adelman said the criteria for judgment should be, "Are the reductions balanced? Are they verifiable? Are they leading toward a safer world?"

White House national security affairs adviser Robert C. McFarlane, in an interview on the CBS Morning News, said the criteria for judgment should be: "Will the Soviet Union maintain a unilateral first-strike capability while the United States has no corresponding capability or not? Will there be Soviet systems excluded from these reductions, but U.S. systems included in an imbalanced way? Will the Soviet Union be entitled to keep building new systems while the U.S. is forbidden to do so?"

Some additional details of the Soviet proposals became known in Washington as the Geneva negotiators discussed them in secret for the first time. The proposed Soviet 50 percent cut in nuclear charges, according to administration officials, would cover missile warheads, bombs aboard bombers, short-range attack missiles and so on.

Sources said Soviet Foreign Minister Eduard Shevardnadze, who delivered the proposal to the White House, said the Soviet Union thinks that the United States has about 12,000 nuclear charges covered by the 50 percent reductions, thus bringing the U.S. total down to 6,000 under the Soviet plan.

The Soviet Union is thought to have fewer such "charges" covered by the Soviet plan. After cuts, the two sides would move to an equal number under the Soviet plan.

Another aspect of the Soviet proposal, sources said, is a percentage limit on the nuclear charges that each side could put on any one segment of its strategic forces. That figure in the Soviet proposal is reported to be 60 percent.

Thus, the Soviets could not put more than 60 percent of their total "charges" in land-based intercontinental missiles, the category in which they have the greatest advantage. U.S. nuclear forces are distributed about evenly between bombers, submarines and land-based missiles.

Under the Soviet plan, administration sources said, Moscow would reduce its force of SS20 missiles in Europe to the size of the independent British and French nuclear force. The Soviets made a similar offer on Intermediate Nuclear Forces (INF), the Euromissile negotiations. This offer, however, evidently would not limit Soviet medium-range missiles based in Soviet Asia, which in the past has been a sticking point for Washington.

A Soviet proposal to ban "new types" of strategic systems might stop several new U.S. missiles and the Stealth bomber, administration sources said. But Washington officials fear that, the way the provision is drawn, it would not limit Soviet forces.